Method and Apparatus for Coded Block Flag Coding in High Efficiency Video Coding

ABSTRACT

A method includes receiving the video bitstream from a media or a processor, recovering a first coded block flag (cbf) from the video bitstream based on a context-based adaptive binary arithmetic coding (CABAC) decoding process according to a first context formation, wherein the first cbf is associated with a first transform unit (TU) of a first color component, and the first cbf indicates whether the first TU of the first color component has at least one non-zero transform coefficient, and recovering a second cbf from the video bitstream based on the CABAC decoding process according to a second context formation, wherein the second cbf is associated with a second TU of a second color component, and the second cbf indicates whether the second TU of the second color component has at least one non-zero transform coefficient.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a Continuation of pending U.S. patentapplication Ser. No. 14/372,696, filed Jul. 16, 2014, which is aNational Stage Application of pending Application No. PCT/CN2013/070160,filed Jan. 7, 2013, which claims priority to PCT Patent Application,Serial No. PCT/CN2012/070612, filed Jan. 19, 2012, entitled “Methods andApparatuses of CBF Coding in HEVC”. The PCT Patent Application is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to video coding. In particular, thepresent invention relates to method and apparatus for coding the cbf(coded block flag) syntax associated with coding unit (CU) and transformunit (TU) in High Efficiency Video Coding (HEVC).

BACKGROUND AND RELATED ART

HEVC (High Efficiency Video Coding) is an advanced video coding systembeing developed under the Joint Collaborative Team on Video Coding(JCT-VC) group of video coding experts from ITU-T Study Group. In HEVCTest Model Version 5.0 (HM-5.0), the inter-coded and intra-codedresidues are coded using block-based transform coding. The blocks(called transform units) are partitioned from a root block (a roottransform unit) using a quad-tree structure. The quad-tree partition isapplied iteratively until a leaf block or a smallest block is reached.Two-dimensional transform is then applied to each of the transformunits. Each TU can be split into four sub-TUs, i.e. leaf TUs. For eachTU, a syntax element named cbf (coded block flag) is transmitted toindicate if the TU has non-zero transformed coefficients or not, where a“1” indicates at least one existing non-zero coefficient and a “0”indicates no non-zero coefficient.

In HM-5.0, the cbf is signaled only for leaf TUs of the residualquad-tree for the luma component. For the chroma components, the cbf issignaled for both the root TU and the leaf TU, however, the cbf is onlysignaled in a TU that is smaller than or equal to the maximum chroma TUsize. FIG. 1 to FIG. 3 illustrate examples of the cbf signaling. In FIG.1, block 110 shows the residual quad-tree splitting of a TU, where aroot TU is partitioned into sub-TUs (TU 0 through TU 6) using quad-treepartition. Block 120 shows the corresponding cbf bits, where TUs 1, 3, 5and 6 have non-zero coefficient and TUs 0, 2 and 4 have no non-zerocoefficient. If the TU is a luma TU, the cbf bits are transmitted onlyfor leaf TUs. An example of cbf signaling (i.e., cbf coding) for a lumaTU is illustrated in FIG. 2A, where four sets of bins “0”, “1” “0101”and “1” correspond to the cbf bits for the four leaves of root TU 210.The cbf bits are signaled in a raster-scan order, i.e., in the order ofupper left TU, upper right TU, lower left TU and lower right TU. For thelower left leaf TU, the TU is further partitioned into four leaf TUs.The cbf bits for this leaf TU are “0101” in the raster-scan order.Accordingly, the four sets of cbf bits 220 are shown in FIG. 2A. Anexample of cbf signaling for a chroma TU is illustrated in FIG. 2B,where the cbf bits are transmitted for both the root TU and the leaf TU.The root TU 230 is partitioned into four leaf TUs and the lower leftleaf TU is further partition into four leaf TUs. Therefore, there arethree levels of cbf bits corresponding to the three levels of TUs. Forthe root TU (i.e., depth=0), cbf bit “1” (indicated by reference number240) is signaled. For the four leaf TUs of the root TU, the cbf bits are“0”, “1”, “1” and “1” (indicated by reference number 250) in theraster-scan order. For the lower left leaf TU, the TU is furtherpartitioned into four leaf TUs with corresponding cbf bits “0”, “1” “0”and “1” (indicated by reference number 260) in the raster-scan order. Asshown in FIG. 2A and FIG. 2B, while the luma TU and chroma TU have thesame RQT (residual quad-tree) structure, the cbf signaling is different.The example in FIG. 2B is for root block smaller than or equal to themaximum chroma TU size. For example, given maximum chroma TU size is16×16 and minimum chroma TU size is 4×4, the size of the root TU 230 is16×16, and the size of each lower left leaf TU is 4×4. When the chromaleaf CU size is larger than the maximum chroma TU size, such as 32×32,there is no cbf signaled in the 32×32 level.

In order to reduce the number of cbf bits, an inferring method is usedfor luma and chroma TUs, where the cbf flag of the fourth leaf TU of aroot TU is inferred by using the cbf flags of other TUs. Therefore, thecbf of the fourth leaf TU does not need to be transmitted.

For luma TUs, the cbf of the fourth leaf TU can be inferred from thecoded block flags (cbfs) of previous three leaf TUs and the cbf of theassociated root TU. Block 310 in FIG. 3 illustrates an example when thecbf of the fourth leaf TU can be inferred. The lower left TU indicatedby thick-lined box 312 is partitioned into four leaf TUs, where the cbfof the fourth leaf TU is 1. Since TU 312 is partitioned into four leafTUs, there is at least one non-zero coefficient among the four leaf TUs.When cbfs of the three previous leaf TUs are all zero (in theraster-scan order), the cbf of the last leaf TU (i.e., the fourth leafTU) must be 1. Therefore, the cbf for the fourth leaf TU in this casecan be inferred. The cbf of a leaf TU is also referred to as a leaf cbffor convenience.

For chroma TUs, the situation is different because cbf is transmittedfor all level of the residual quad-tree. For the four leaf TUsassociated with each root TU, the cbf for the root TU is transmitted. Ifthe cbf of the TU is 1 (block 312 in FIG. 3), there must be at least onenon-zero leaf TU among the four leaf TUs. Therefore, if the cbfs of thefirst three leaf TUs are all zero, the cbf of the last TU (indicated bya circle) must be 1. In this case, the last cbf can be inferred and doesnot need to be signaled. Moreover, the inferring mechanism can beapplied to both intra and inter coded TU for the chroma component.

In HEVC, there is also a root residual flag for an inter-coded codingunit (CU). When residual flag is false, there is no need to signal allthe cbfs for Y, U and V components. When the residual flag is true andTU depth of current CU is 0, the luma cbf can be inferred to be 1 ifchroma cbfs are all 0. Therefore, if the cbfs for U (block 320) and V(block 330) are all 0, the cbf for the luma TU at depth 0 is inferred tobe 1 as shown in FIG. 3.

In HM5.0, the maximum TU size is 16×16 for the chroma component and32×32 for the luma component. However, the maximum CU size is 32×32 forthe chroma component. Therefore, the maximum CU size and TU size are notthe same. Furthermore, in HM-5.0, the chroma cbf is signaled for the TUwith a size smaller or equal to the maximum TU size. For example, whenthe CU size is 64×64, i.e. chroma CU size is 32×32, the maximum TU sizecorresponds to 16×16. Therefore, four root cbfs will be transmitted forthe four 16×16 chroma TUs of this 32×32 CU. In this case, even when thefour cbfs are all 0, the cbfs will be transmitted, as illustrated inFIG. 4, where the size of the chroma CU 410 is 32×32.

As mentioned above, the cbf signaling method is different for the lumaTU and chroma TU. It is desirable to use a unified cbf signaling methodto simplify the process. In addition, the existing cbf signaling methodhas some redundancy and it is desirable to further improve theefficiency of the existing cbf signaling method.

BRIEF SUMMARY OF THE INVENTION

A method and apparatus for encoding and decoding of a video bitstreamare disclosed. In one embodiment, receives the video bitstream from amedia or a processor, recovers a first coded block flag (cbf) from thevideo bitstream based on a context-based adaptive binary arithmeticcoding (CABAC) decoding process according to a first context formation,wherein the first cbf is associated with a first transform unit (TU) ofa first color component, and the first cbf indicates whether the firstTU of the first color component has at least one non-zero transformcoefficient, and recovers a second cbf from the video bitstream based onthe CABAC decoding process according to a second context formation,wherein the second cbf is associated with a second TU of a second colorcomponent, and the second cbf indicates whether the second TU of thesecond color component has at least one non-zero transform coefficient.The first color component is different from the second color component,and the first context formation and the second context formation bothdepend on depth of residual quad-tree (RQT).

In another embodiment, a method receives a first transform unit (TU) ofa first color component and a second TU of a second color component froma media or a processor, determines a first residual quad-tree (RQT)associated with the first TU and a second RQT associated with the secondTU, and determines a first coded block flag (cbf) for the first TU ofthe first color component and a second cbf for the second TU of thesecond color component, wherein the first cbf indicates whether thefirst TU of the first color component has at least one non-zerotransform coefficient and the second cbf indicates whether the second TUof the second color component has at least one non-zero transformcoefficient. The method further generates a video bitstream by encodingthe first cbf based on a context-based adaptive binary arithmetic coding(CABAC) encoding process according to a first context formation andencoding the second cbf based on the CABAC decoding process according toa second context formation. The first color component is different fromthe second color component, and the first context formation and thesecond context formation both depend on depth of RQT.

In another embodiment, an apparatus includes one or more electroniccircuits or processors arranged to: receive the video bitstream from amedia or a processor; recover a first coded block flag (cbf) from thevideo bitstream based on a context-based adaptive binary arithmeticcoding (CABAC) decoding process according to a first context formation,wherein the first cbf is associated with a first transform unit (TU) ofa first color component, and the first cbf indicates whether the firstTU of the first color component has at least one non-zero transformcoefficient; and recover a second cbf from the video bitstream based onthe CABAC decoding process according to a second context formation,wherein the second cbf is associated with a second TU of a second colorcomponent, and the second cbf indicates whether the second TU of thesecond color component has at least one non-zero transform coefficient.The first color component is different from the second color component,and the first context formation and the second context formation bothdepend on depth of residual quad-tree (RQT).

In another embodiment, an apparatus includes one or more electroniccircuits or processors arranged to: receive a first transform unit (TU)of a first color component and a second TU of a second color componentfrom a media or a processor; determine a first residual quad-tree (RQT)associated with the first TU and a second RQT associated with the secondTU, determine a first coded block flag (cbf) for the first TU of thefirst color component and a second cbf for the second TU of the secondcolor component, wherein the first cbf indicates whether the first TU ofthe first color component has at least one non-zero transformcoefficient and the second cbf indicates whether the second TU of thesecond color component has at least one non-zero transform coefficient,and generate a video bitstream by encoding the first cbf based on acontext-based adaptive binary arithmetic coding (CABAC) encoding processaccording to a first context formation and encoding the second cbf basedon the CABAC decoding process according to a second context formation.The first color component is different from the second color component,and the first context formation and the second context formation bothdepend on depth of RQT.

In another embodiment, a non-transitory computer readable medium storesa computer-executable program, the computer-executable program, whenexecuted, causing a decoder to perform the following steps: receivingthe video bitstream from a media or a processor; recovering a firstcoded block flag (cbf) from the video bitstream based on a context-basedadaptive binary arithmetic coding (CABAC) decoding process according toa first context formation, wherein the first cbf is associated with afirst transform unit (TU) of a first color component, and the first cbfindicates whether the first TU of the first color component has at leastone non-zero transform coefficient; and recovering a second cbf from thevideo bitstream based on the CABAC decoding process according to asecond context formation, wherein the second cbf is associated with asecond TU of a second color component, and the second cbf indicateswhether the second TU of the second color component has at least onenon-zero transform coefficient. The first color component is differentfrom the second color component, and the first context formation and thesecond context formation both depend on depth of residual quad-tree(RQT).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the residual quad-tree structure andthe coded block flags of the leaf TUs.

FIG. 2A illustrates an example of coded block flag signaling method forthe luma TU according to HM-5.0.

FIG. 2B illustrates an example of coded block flag signaling method forthe chroma TU according to HM-5.0.

FIG. 3 illustrates an example of coded block flag signaling based oninferring of the luma TUs and chroma TUs.

FIG. 4 illustrates an example of coded block flag signaling for the cbfsof four 16×16 chroma root TUs.

FIG. 5 illustrates an example of coded block flag inferring mechanismfor an inter CU according to an embodiment of the present invention.

FIG. 6A and FIG. 6B illustrate examples where the cbf of the chromacomponent is signaled at CU level according to an embodiment of thepresent invention.

FIG. 7 illustrates an exemplary flowchart of an encoder incorporating anembodiment of the present invention.

FIG. 8 illustrates an exemplary flowchart of a decoder incorporating anembodiment of the present invention.

FIG. 9 illustrates an exemplary flowchart of an encoder incorporatinganother embodiment of the present invention.

FIG. 10 illustrates an exemplary flowchart of a decoder incorporatinganother embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the present invention, luma and chroma cbfsignaling methods are unified by extending the chroma cbf coding methodto the luma cbf. Therefore, the luma and chroma cbfs are both signaledfor each level of the residual quad tree. In other words, cbf signalingis performed for both the root TU and the leaf TU. Inferring methods forthe luma and chroma components are also unified in this case.Accordingly, the luma TU uses the same inferring method as the chromaTU. In other words, if the cbfs of the first three leaf TUs are allzero, the cbf of the last TU must be 1.

In another embodiment, the residual flag inferring method for the interCU is also applied to the unified signaling methods. Therefore, when theresidual flag is true and the cbfs for the chroma TUs are all 0, the cbfof the top root luma TU is inferred to be 1 regardless of whether thetop root TU is further split or not. Furthermore, this residual flaginferring method for the inter CU can be applied to other TU depths inaddition to depth 0. In other words, when the TU is further split andchroma cbfs are all zero, the cbf of the luma TU can be inferred tobe 1. As illustrated in FIG. 5, when the residual flag is 1 and the cbfsfor chroma (U 520 and V 530) root TUs are all 0, the cbf of the lumaroot TU 510 can be inferred to be 1.

Furthermore, the context formation of the luma cbf can also be unifiedwith the chroma cbf so that context formation for cbf coding based onCABAC (context-based adaptive binary arithmetic coding) is dependent onthe TU depth for both the luma and chroma components. In order to reducethe complexity of entropy coding of cbf flag, the number of contexts canbe reduced. Furthermore, bypass coding mode can be used for CABAC-basedcbf coding.

In another embodiment, the root cbf is always signaled at the CU levelregardless of the size of the maximum TU. Therefore, there is always aroot cbf in each CU. FIG. 6A and FIG. 6B illustrate examples of the cbfcoding process when the chroma CU size is 32×32 and the maximum TU sizeis 16×16. In FIG. 6A, the chroma CU corresponds to a 32×32 block, whichis larger than the maximum chroma TU size (i.e., 16×16). The root cbffor the chroma CU is 0 since all the chroma TUs associated with the CUhave no non-zero coefficient as indicated by 0. Since a root cbf in eachCU is always signaled according to an embodiment of the presentinvention, a 0 will be signaled for the CU and there is no need foradditional cbf signaling. FIG. 6B illustrates another example, where thelower left TU contains at least one non-zero coefficient. In this case,a 1 is signaled for the root chroma CU and additional cbf bits “0 0 1 0”are signaled to indicate which TU contains non-zero coefficients. Themaximum TU sizes for the luma and chroma components are known for acoding system based on HM-5.0. The information of the maximum TU sizemay also be signaled in the bitstream, such as in the sequence level(e.g., SPS) of the bitstream.

In yet another embodiment, luma and chroma cbf signaling methods areunified by extending the luma cbf coding method to the chroma cbf. As aresult, the luma and chroma cbf are both signaled only for the leaf TUs.

The cbf signaling method described above can be used in a video encoderas well as a video decoder. FIG. 7 illustrates an exemplary flowchart ofan encoder incorporating an embodiment of the present invention. Theresidues of a current CU are determined as shown in step 710, where thecurrent CU size is larger than the maximum TU size. A first cbf of acolor component indicating whether the current CU (depth=0) has at leastone non-zero transform coefficient is determined as shown in step 720.According to the result of the first cbf, different processing routesare taken as shown in step 730. If the current CU of the color componenthas at least one non-zero transform coefficient, four second cbfs of thecolor component, each indicating whether one of four sub-blocks(depth=1) of the color component in the current CU has at least onenon-zero transform coefficient, are determined as shown in step 740. Inthis case, both the first cbf and the four second cbfs are incorporatedinto the video bitstream as shown in step 750. If the current CU has nonon-zero transform coefficient, only the first cbf is incorporated intothe video bitstream as shown in step 760. The cbf signaling byincorporating the cbf in the video bitstream will allow a decoder torecover the residual quad-tree structure and perform decoding processaccordingly. In some embodiments, if at least one of the sub-block ofthe color component has at least one non-zero transform coefficient andthe sub-block does not reach the minimum TU size of the color component,the sub-block(s) with non-zero transform coefficient(s) is furtherpartitioned into four leaf blocks (depth=2). Four third cbfs of thecolor component, each indicating whether one of the four leaf blocks ofthe color component has at least one non-zero transform coefficient, aredetermined for each sub-block with non-zero transform coefficient. Thefour third cbfs of the color component are also incorporated into thevideo bitstream. The sub-blocks and leaf blocks may be root TUs and leafTUs in the current CU. The color component may be luma or chromacomponent.

FIG. 8 illustrates an exemplary flowchart of a decoder incorporating anembodiment of the present invention. The video bitstream is receivedfrom a media or a processor as shown in step 810. The video bitstreammay be stored in a media such as a storage media (hard drive, opticaldisc, or flash card) or computer memory (RAM, PROM, DRAM or flashmemory). The video bitstream may also be received and/or processed by aprocessor. For example, in a broadcast environment, a channel receivermay receive modulated signal, demodulate and de-multiplex to recover adesired bitstream. In this case, the video bitstream is received from aprocessor (i.e., the channel receiver). In step 830, a first cbf of acolor component indicating whether the current CU (depth=0) of the colorcomponent has at least one non-zero transform coefficient is decoded.According to the decoding result, different decoding routes are taken asshown in step 840. If the first cbf of the color component is not zero,four second cbfs of the color component, each indicating whether one offour sub-blocks (depth=1) of the color component in the current CU hasat least one non-zero transform coefficient, are decoded as shown instep 850. The residual quad-tree structure of the current CU of thecolor component is then determined based on the first cbf and foursecond cbfs as shown in step 860. If the four first cbfs of the colorcomponent are zero, the residual quad-tree structure of the current CUof the color component is then determined based on the first cbf only asshown in step 870. In some embodiments, four third cbfs of the colorcomponent are also decoded if one of the sub-block of the colorcomponent at depth=1 has at least one non-zero transform coefficient andthe sub-block is larger than the minimum TU size of the color component.Each of the four third cbfs of the color component indicates whether oneof four leaf blocks of the color component has at least one non-zerotransform coefficient. The sub-blocks and the leaf blocks may be rootTUs and leaf TUs in the current CU. The color component may be chroma orluma component.

FIG. 9 illustrates an exemplary flowchart of an encoder incorporatinganother embodiment of the present invention. In step 910, a TU isreceived from a media or a processor. The RQT (residual quad-tree)associated with the TU is then determined as shown in step 920. One ormore cbfs corresponding to the RQT of the TU are determined in step 930,wherein signaling of the cbf is the same for the luma component and thechroma component.

FIG. 10 illustrates an exemplary flowchart of a decoder incorporatinganother embodiment of the present invention. In step 1010, the videobitstream is received from a media or a processor. A cbf associated witha TU is decoded in step 1020, wherein the cbf is recovered from thevideo bitstream. The residual quad-tree structure of the TU isdetermined based on the cbf as shown in step 1030, wherein signaling ofthe cbf is the same for the luma component and the chroma component.

The flowcharts shown above are intended to illustrate examples of cbfsignaling for a video encoder and a decoder incorporating embodiments ofthe present invention. A person skilled in the art may modify each step,re-arranges the steps, split a step, or combine steps to practice thepresent invention without departing from the spirit of the presentinvention.

The above description is presented to enable a person of ordinary skillin the art to practice the present invention as provided in the contextof a particular application and its requirement. Various modificationsto the described embodiments will be apparent to those with skill in theart, and the general principles defined herein may be applied to otherembodiments. Therefore, the present invention is not intended to belimited to the particular embodiments shown and described, but is to beaccorded the widest scope consistent with the principles and novelfeatures herein disclosed. In the above detailed description, variousspecific details are illustrated in order to provide a thoroughunderstanding of the present invention. Nevertheless, it will beunderstood by those skilled in the art that the present invention may bepracticed.

Embodiment of the present invention as described above may beimplemented in various hardware, software codes, or a combination ofboth. For example, an embodiment of the present invention can be acircuit integrated into a video compression chip or program codeintegrated into video compression software to perform the processingdescribed herein. An embodiment of the present invention may also beprogram code to be executed on a Digital Signal Processor (DSP) toperform the processing described herein. The invention may also involvea number of functions to be performed by a computer processor, a digitalsignal processor, a microprocessor, or field programmable gate array(FPGA). These processors can be configured to perform particular tasksaccording to the invention, by executing machine-readable software codeor firmware code that defines the particular methods embodied by theinvention. The software code or firmware code may be developed indifferent programming languages and different formats or styles. Thesoftware code may also be compiled for different target platforms.However, different code formats, styles and languages of software codesand other means of configuring code to perform the tasks in accordancewith the invention will not depart from the spirit and scope of theinvention.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described examples areto be considered in all respects only as illustrative and notrestrictive. The scope of the invention is therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

1. A method for decoding a video bitstream, the method comprising:receiving the video bitstream from a media or a processor; recovering afirst coded block flag (cbf) from the video bitstream based on acontext-based adaptive binary arithmetic coding (CABAC) decoding processaccording to a first context formation, wherein the first cbf isassociated with a first transform unit (TU) of a first color component,and the first cbf indicates whether the first TU of the first colorcomponent has at least one non-zero transform coefficient; andrecovering a second cbf from the video bitstream based on the CABACdecoding process according to a second context formation, wherein thesecond cbf is associated with a second TU of a second color component,and the second cbf indicates whether the second TU of the second colorcomponent has at least one non-zero transform coefficient, wherein thefirst color component is different from the second color component, andwherein the first context formation and the second context formationboth depend on depth of residual quad-tree (RQT).
 2. The method of claim1, wherein the first color component is luma component, and the secondcolor component is chroma component.
 3. The method of claim 1, whereinthe first context formation depends on depth of RQT associated with thefirst TU.
 4. The method of claim 1, wherein the second context formationdepends on depth of RQT associated with the second TU.
 5. The method ofclaim 1, wherein one of the first cbf and the second cbf is signaled ata root TU and leaf TUs, and another one of the first cbf and the secondcbf is signaled at leaf TUs and not signaled at a root TU.
 6. The methodof claim 1, wherein at least one of the first cbf and the second cbf issignaled at a root level of a coding unit (CU) regardless whether blocksize of the CU is larger than a maximum TU size.
 7. A method forencoding a coded block flag, the method comprising: receiving a firsttransform unit (TU) of a first color component and a second TU of asecond color component from a media or a processor; determining a firstresidual quad-tree (RQT) associated with the first TU and a second RQTassociated with the second TU; and determining a first coded block flag(cbf) for the first TU of the first color component and a second cbf forthe second TU of the second color component, wherein the first cbfindicates whether the first TU of the first color component has at leastone non-zero transform coefficient and the second cbf indicates whetherthe second TU of the second color component has at least one non-zerotransform coefficient; generating a video bitstream by encoding thefirst cbf based on a context-based adaptive binary arithmetic coding(CABAC) encoding process according to a first context formation andencoding the second cbf based on the CABAC decoding process according toa second context formation, wherein the first color component isdifferent from the second color component, wherein the first contextformation and the second context formation both depend on depth of RQT.8. The method of claim 7, wherein the first color component is lumacomponent, and the second color component is chroma component.
 9. Themethod of claim 7, wherein the first context formation depends on depthof RQT associated with the first TU.
 10. The method of claim 7, whereinthe second context formation depends on depth of RQT associated with thesecond TU.
 11. The method of claim 7, wherein one of the first cbf andthe second cbf is signaled at a root TU and leaf TUs, and another one ofthe first cbf and the second cbf is signaled at leaf TUs and notsignaled at a root TU.
 12. The method of claim 7, wherein at least oneof the first cbf and the second cbf is signaled at a root level of acoding unit (CU) regardless whether block size of the CU is larger thana maximum TU size.
 13. An apparatus for decoding a video bitstream, theapparatus comprising one or more electronic circuits or processorsarranged to: receive the video bitstream from a media or a processor;recover a first coded block flag (cbf) from the video bitstream based ona context-based adaptive binary arithmetic coding (CABAC) decodingprocess according to a first context formation, wherein the first cbf isassociated with a first transform unit (TU) of a first color component,and the first cbf indicates whether the first TU of the first colorcomponent has at least one non-zero transform coefficient; and recover asecond cbf from the video bitstream based on the CABAC decoding processaccording to a second context formation, wherein the second cbf isassociated with a second TU of a second color component, and the secondcbf indicates whether the second TU of the second color component has atleast one non-zero transform coefficient, wherein the first colorcomponent is different from the second color component, and wherein thefirst context formation and the second context formation both depend ondepth of residual quad-tree (RQT).
 14. An apparatus for encoding a codedblock flag, the apparatus comprising one or more electronic circuits orprocessors arranged to: receive a first transform unit (TU) of a firstcolor component and a second TU of a second color component from a mediaor a processor; determine a first residual quad-tree (RQT) associatedwith the first TU and a second RQT associated with the second TU; anddetermine a first coded block flag (cbf) for the first TU of the firstcolor component and a second cbf for the second TU of the second colorcomponent, wherein the first cbf indicates whether the first TU of thefirst color component has at least one non-zero transform coefficientand the second cbf indicates whether the second TU of the second colorcomponent has at least one non-zero transform coefficient; generating avideo bitstream by encoding the first cbf based on a context-basedadaptive binary arithmetic coding (CABAC) encoding process according toa first context formation and encoding the second cbf based on the CABACdecoding process according to a second context formation, wherein thefirst color component is different from the second color component,wherein the first context formation and the second context formationboth depend on depth of RQT.
 15. A non-transitory computer readablemedium storing a computer-executable program, the computer-executableprogram, when executed, causing a decoder to perform the followingsteps: receiving the video bitstream from a media or a processor;recovering a first coded block flag (cbf) from the video bitstream basedon a context-based adaptive binary arithmetic coding (CABAC) decodingprocess according to a first context formation, wherein the first cbf isassociated with a first transform unit (TU) of a first color component,and the first cbf indicates whether the first TU of the first colorcomponent has at least one non-zero transform coefficient; andrecovering a second cbf from the video bitstream based on the CABACdecoding process according to a second context formation, wherein thesecond cbf is associated with a second TU of a second color component,and the second cbf indicates whether the second TU of the second colorcomponent has at least one non-zero transform coefficient, wherein thefirst color component is different from the second color component, andwherein the first context formation and the second context formationboth depend on depth of residual quad-tree (RQT).